This invention relates to a to a method of providing solder or brazing barriers with a photolithographic process in conjunction with a refractory or nonsolderable metal deposit in order to achieve and alloy or solder barrier. It has a particular application to thin brittle wafers having vias such as a semiconductor chip or a substrate.
Semiconductor devices such as thin film network components, gallium arsenide MMICs (monolithic microwave integrated circuits) and discrete field effect transistors commonly employ vias (feed through holes in the device) which serve as conduits for connecting a backside ground plane to the front side circuit metalization.
FIG. 1 illustrates this arrangement in which a semiconductor chip 10, which in the embodiment of FIG. 1 is a gallium arsenide device, has a ground plane established by a metal deposit 11 that is deposited on the backside 15 of the semiconductor chip 10. A circuit configuration is provided on the front side 17 of the semiconductor chip 10 in the form of a metalization 18. The metalization 18 is connected to the ground plane 11 by a via 14 that serves as a conduit for connecting the ground plane 11 to the front side metalization 18 at point 19.
Referring to FIG., 2 the vias in the semiconductor chip 10 are essential to high performance MMICs. These MMICs in complex circuit arrangements are connected to additional circuits which may generate substantial heat when in operation. Therefore, for these complex circuits MMIC's are mounted to a substrate such as that illustrated in FIG. 2 and denoted as mounting block 21. This is very often required to achieve good thermal characteristics. However, when the semiconductor device 10 is mounted to the substrate 21 through the use of soldering, the solder 23 tends to be drawn up into the via 14 as indicated at 25 filling all or most of the via 14 as is shown in FIG. 2. The differential expansion between the solder and the brittle semiconductor chip 10 such as gallium arsenide or a ceramic thin film can cause the material near the via to crack, as is illustrated at 27. Such cracks can cause either immediate device failures or propagate under operating stress and produce future device failures. Altough such cracks do not always develop, experience with MMIC devices, as well as other semiconductor devices and ceramic thin films, illustrate this effect to be very serious in both yield and reliability.